In many localities, a plurality of land mobile radio frequency channels are allocated for communication among mobile radio transceivers. Generally, there may be, and usually are, many more mobile radio transceivers than there are available radio channels in a given locality. Each of these radio channels usually includes at least two separate and distinct frequencies, one for transmitting and the other for receiving.
Consequently, some arrangement is needed for enabling a mobile radio transceiver user to obtain access to a radio channel that is not in use (without disturbing the privacy of channels already in use) and to thereafter establish communication with one or more other specifically designated mobile transceivers over that acquired radio frequency channel.
One such arrangement for establishing communication between a first mobile radio station and a second mobile radio station through a repeater over a radio frequency channel selected from a plurality of such channels is disclosed in commonly-assigned U.S. Pat. No. 4,360,927 to Bowen et al (issued Nov. 23, 1982). The disclosure of U.S. Pat. No. 4,360,927 is expressly incorporated herein by reference.
Bowen et al discloses a positive handshake between a mobile station originating a call and a repeater operating on a vacant channel. The operation of the arrangement disclosed by Bowen et al can be explained in connection with FIG. 1 of the present application (which is a graphical illustration of the channel acquisition protocol used in the prior art Bowen et al system).
As depicted in FIG. 1, a mobile station desirous of originating a transmission searches a plurality of channels for an idle channel (indicated by the absence of a busy signal or tone). When the mobile station locates an idle channel, it transmits a busy signal burst which is received by the repeater operating on the idle channel. The repeater, upon detecting the busy signal burst transmitted by the mobile station, transmits an acquisition signal burst which preferably has different characteristics from the busy signal. The acquisition signal burst is transmitted only if the channel is in fact vacant.
The Bowen et al type mobile station (as depicted in FIG. 1) originating the transmission listens for the acquisition signal, and will complete the channel acquisition signalling sequence only after detecting the acquisition signal burst returned from the repeater. If detected, the mobile station then completes the channel acquisition protocol by transmitting a first tone followed by a second tone (i.e. a "group signal" or "collect" tone followed by a "sub-group signal") which collectively identify a particular one or ones of a plurality of mobile transceivers with which the user would like to communicate.
The repeater in the FIG. 1 system receives and re-transmits the called station identifying tones to alert other designated mobile stations (which if in an "idle" mode are continuously scanning the channels to detect a preassigned calling signal sequence) identified by the transmitted signalling sequence that communication is to be established. The repeater simultaneously transmits a busy signal throughout the channel acquisition signalling sequence (beginning at the time that a busy signal transmitted by the mobile station is received by the repeater) to ensure that other mobile stations searching for an idle channel detect that the channel is being used.
When the mobile station of FIG. 1 has completed transmission of the desired called signal sequence, it switches back to a receive mode to listen for a busy signal still being transmitted by the repeater. If such a busy signal is still present, the mobile station typically assumes that the communication channel has been successfully acquired and dedicated to its use, and so alerts a user by then switching into a "ready" mode. The user may then utilize a microphone to transceive voice information over the acquired communications channel via the repeater to other mobile transceivers which have been alerted by the calling signal sequence. On the other hand, if the mobile transceiver does not hear a busy signal at the conclusion of its calling signal transmission, it resumes scanning of the communication channels until another idle channel is found, and attempts to acquire this idle channel.
Although quite successful in its own right, the exemplary arrangement disclosed by Bowen et al can be further improved in at least two ways. (It should be noted throughout that the discussion herein of the Bowen et al type system refers only to the exemplary embodiment disclosed in the Bowen et al patent and does not relate to the scope of any patent claims in the Bowen et al patent.) First, a mobile transceiver in the Bowen et al system can "false" on voice information present on a scanned communication channel (i.e. it can mistake voice information for signalling information and mistakenly determine that it has been called). This effect sometimes has been observed in the field and is believed to be caused by a mobile station detecting, as channel acquisition signalling tones, certain spectral components sequentially present in voice transmissions. Falsing on voice information is particularly prone to occur when two pre-assigned signalling tones are relatively close to one another in frequency, but may occur for any assigned pairs of identifying tones. Such falsing is not merely a nuisance (since it causes a mobile station to alert a user that a call has come in and begins receiving and producing audio of the voice transmission which caused the falsing), but also degrades the security of the entire system. Ways of eliminating voice falsing (such as by transmitting signalling information on frequencies outside of the voice band) are known but are complex and expensive.
Another possible problem with a two-tone Bowen et al type system is the relatively small number of different groups of mobile stations which can be served by a single group of repeaters due to the limited number of signalling tones used. The signalling (group identifying) tones used in the Bowen et al system are changed in frequency to select different sets or groups of mobile stations (or even a single station). It has been found that such different frequencies must be relatively well separated from one another in the frequency domain to insure reliable detection and discrimination between tones (e.g. if relatively inexpensive filters and other active signal processing devices which are small enough to be included in a mobile radio transceiver are to be used). To provide adequate frequency separation between different signalling tones, the maximum number of different unique tones is limited (by the overall bandwidth limitations of the transmissions) to a predetermined number (e.g. approximately 34). Using 34 different signalling tone frequencies, the maximum number of groups which can utilize a single repeater system is thus approximately 34.times.33 (about 1,100). Unfortunately, it has been found in practice that the actual number of groups which can be supported on a single system is somewhat less because of the need to restrict the use of adjacent tones due to increased probability of falsing.
Other prior art systems provide for the "marking" of unused ones of a plurality of available channels. U.S. Pat. No. 3,173,966 to Rypinski (1965),and an IMTS Telephone Service article (designated LBI-8748A) published by General Electric Company teach a multiple channel radio telephone system wherein only a selected one of the available channels is "marked" as an idle channel at any given time but wherein all channels are utilized for passing both control and communication information (there is no single dedicated control channel). Also taught is an interchange of control signals between a mobile unit and a central station before transceive operations are permitted. U.S. Pat. No. 3,707,679 to Bruley et al (1972) also teaches an automatic radio telephone system wherein a selected free channel is "marked" and control signals are interchanged before transceive operations are permitted. See also the following references disclosing the "marking" of idle ones of a plurality of available channels:
U.S. Pat. No. 3,535,636 to Muilwijk (1970)
U.S. Pat. No. 4,009,442 to von Bromssen (1977)
U.S. Pat. No. 3,376,509 to Willcox et al (1968) and
U.S. Pat. No. 3,555,424 to Malm (1971).
Some other references which disclose the broad concept of selecting one of a plurality of available channels for communication are:
U.S. Pat. No. 4,013,958 to Spayth (1977)
U.S. Pat. No. 4,125,808 to Graham (1978)
U.S. Pat. No. 4,129,749 to Goldman (1978)
U.S. Pat. No. 3,808,537 to Sarati et al (1974)
U.K. Pat. No. 850,698 (1960)
U.K. Pat. No. 1,145,899 (1969)
U.K. Pat. No. 1,175,130 (1969)
U.K. Pat. No. 1,185,789 (1970)
U.K. Pat. No. 1,276,941 (1972)
U.K. Pat. No. 1,297,071 (1972)
German Pat. No. 2,030,347 (1978) and
Rybicki et al, The Basics of Trunked Mobile Radio (Mobile Times, October, 1980).